function pfn_page_default_image(){
  return get_template_directory_uri() . '/assets/pagina-hardnieuws.jpg';
}

function pfn_page_image_url(){
  if (is_page() && has_post_thumbnail()) {
    return get_the_post_thumbnail_url(get_the_ID(), 'full');
  }
  $custom = get_theme_mod('page_header_image');
  if ($custom) {
    return $custom;
  }
  return pfn_page_default_image();
}

function pfn_page_photo_customizer($wp_customize){
  $wp_customize->add_section('pfn_page_photos', array(
    'title' => 'Vervolgpagina bovenfoto',
    'priority' => 26
  ));

  $wp_customize->add_setting('page_header_image');
  $wp_customize->add_control(new WP_Customize_Image_Control($wp_customize, 'page_header_image', array(
    'label' => 'Standaard foto boven vervolgpagina’s',
    'section' => 'pfn_page_photos',
    'settings' => 'page_header_image'
  )));
}
add_action('customize_register','pfn_page_photo_customizer');
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="https://www.persfotonh.nl/wp-sitemap-index.xsl" ?>
<sitemapindex xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"><sitemap><loc>https://www.persfotonh.nl/wp-sitemap-posts-post-1.xml</loc></sitemap><sitemap><loc>https://www.persfotonh.nl/wp-sitemap-posts-page-1.xml</loc></sitemap><sitemap><loc>https://www.persfotonh.nl/wp-sitemap-taxonomies-category-1.xml</loc></sitemap><sitemap><loc>https://www.persfotonh.nl/wp-sitemap-users-1.xml</loc></sitemap></sitemapindex>